Thermal bonding conjugate fibers can be processed by heat fusion bonding utilizing thermal energy such as hot air or a heated roll and the like, and these fibers can be widely used for hygiene supplies such as diapers, napkins, pads, etc., articles for daily living, or industrial supply materials such as filters and the like because bulkiness and softness are easily obtained thereby. Bulkiness and softness are extremely important, especially in hygiene supplies because they are items in direct contact with the human skin and because body fluids such as urine, menstrual flow, and the like must be quickly absorbed thereby. Typical means of obtaining bulkiness involve using a highly rigid resin or using a fiber with increased fineness, but in such cases the softness thereof is decreased, and the physical irritation toward the skin is increased. On the other hand, when softness is given priority to control the irritation of the skin, a nonwoven fabric with inferior body fluid absorption results because bulkiness, and especially the cushioning effect with respect to body weight, is markedly decreased.
As a result, many methods have been proposed for obtaining a fiber and nonwoven fabric that has both bulkiness and softness. For example, in Japanese Patent Application Publication No. S63-135549, a process for producing a nonwoven fabric with a high level of bulkiness has been disclosed involving a sheath-core conjugate fabric wherein a polypropylene with a high degree of isotacticity forms the core member, and a resin comprising mainly polyethylene forms the sheath member. This process is one imparting bulkiness to the obtained nonwoven fabric by using a highly rigid resin for the core member of the conjugate fiber, but the softness thereof is unsatisfactory; moreover, the bulkiness of the nonwoven fabric obtained thereby is decreased, especially if the thermal bonding temperature is high, making it almost impossible to obtain both properties with this process.
For example, in Japanese Patent Application Publication No. 2000-336526 and Japanese Patent published and examined Application No. H3-21648, methods have been proposed for imparting bulkiness using a polyester in the core member and polyethylene or polypropylene in the sheath member. In the case of Japanese Patent Application Publication No. 2000-336526, a sheath-core conjugate fiber using polyolefin for the sheath member and a polyester with a melting point ≧20° C. higher than that of the aforementioned polyolefin for the core member is heat-treated after drawing and crimping, and the heat treatment is performed with hot air at a temperature ≧10° C. higher than the glass transition temperature of the aforementioned polyester, but lower by 20° C. or more than the melting point of the aforementioned polyolefin to impart softness and bulkiness to a nonwoven fabric. However, when the fiber is fabricated into a nonwoven fabric, during the process of performing thermal bonding at a temperature equal to or greater than the melting point of the polyolefin, a decrease in thickness occurs due to relaxation of the crimp, shrinkage, and the like because the crimped form is not sufficiently stable with respect to heat, and it is almost impossible to obtain a bulky nonwoven fabric.
In the case of Japanese Patent published and examined Application No. H3-21648, on the other hand, bulkiness is imparted to a nonwoven fabric by using a polyethylene or polypropylene for the latently adhesive component and a polyester for the other, and a conditioning heat treatment is performed at a preselected temperature range after drawing and crimping, and although bulkiness was superb in this case, the softness of the nonwoven fabric obtained thereby was insufficient. In addition, because relaxation of the crimping sometimes occurs in the conditioning step of this method, the crimped form was still lacking in stability.